Large two-stroke uniflow turbocharged compression-ignited internal combustion crosshead engines are typically used in propulsion systems of large ships or as prime mover in power plants. The sheer size, weight and power output renders them completely different from common combustion engines and places large two-stroke turbocharged compression-ignited internal combustion engines in a class for themselves.
Large two-stroke turbocharged compression-ignited internal combustion engines of the crosshead type are typically used in propulsion systems of large ships or as prime mover in power plants.
Large two-stroke compression-ignited internal combustion engines are conventionally operated with a liquid fuel such as e.g. fuel oil or heavy fuel oil but increased focus on environmental aspects has led to the development towards using alternative types of fuel such as gas, methanol, coal slurry, petroleum coke and the like. One group of fuels that is in increasing demand are low flashpoint fuels.
Many low flashpoint fuels, such as methanol, ethanol, LPG, DME or biofuel, naphta, gasoline (petrol), crude gasoline, crude oil are relatively clean fuels that result in significantly lower levels of sulfurous components, NOx and CO2 in the exhaust gas when used as fuel for a large low-speed uniflow turbocharged two-stroke internal combustion engine when compared with e.g. using heavy fuel oil as fuel.
However, there are problems associated with using a low flashpoint fuels in a large low-speed uniflow turbocharged two-stroke internal combustion engine. One of those problems is the low flashpoint, which causes significant problems if low flashpoint fuel leaks into one of the other systems of the engine and mixes with another fluid, such as e.g. the lubrication oil system. Low flashpoint fuel, is inherently easy to ignite and vapors thereof can easily form explosive mixtures. Thus, should low flashpoint find its way into another system of the engine it is necessary to stop the engine operation for safety reasons and to clean or replace all of the liquid in such a system, a costly and cumbersome affair for the operator of the engine.
The timing of the fuel injection highly affects the combustion pressure in a Diesel engine (compression-ignited engine) and therefore the timing of the fuel injection in a compression-ignited engine needs to be controlled very accurately.
It is known in the art to provide large two-stroke compression ignited internal combustion engines with a common rail type system that stores and distributes the gas at the required injection pressure of typically several hundred bar (depending on the type of gas and the engine requirements), with accumulators close to the fuel valves. The common rail type system is connected to two or three fuel injection valves in the cylinder cover of each cylinder. The fuel injection valves are electronically controlled and fuel injection is timed by electronically (the signal originates in an electronic control unit but the actual signal to the fuel valve is typically a hydraulic signal, i.e. electronic signal is converted to a hydraulic signal between the electronic control unit and the fuel valve) controlling the time (relative to the engine cycle) at which the fuel injection valve is opened.
The amount of fuel admitted to a cylinder in one injection event is electronically controlled by the length of the time interval from the opening of the fuel valve to the closing of the fuel valve. In order to ensure safety against ill-timed and/or unlimited injection the so-called window valve is provided between the accumulator and the fuel valve. Thus the maximum amount of fuel that can be injected when e.g. a fuel valve is stuck in its open position is the amount of fuel gas that is present in the system between the window valve and the fuel valve, which is a relatively small and therefore safe amount.
The known common rail type gaseous fuel supply system for large two-stroke compression-ignited internal combustion engines have disadvantages when operating on LPG or any other similar low flashpoint fuel with a relatively high compressibility. The injection pressure for LPG needs to be as high as 600 bar, which means that the common rail system including all valves, accumulators, pipes, etc., needs be laid out for this high pressure. Furthermore, the safety concept with the window valves is not well suited for dense gas like LPG, since firstly the gas channels between window valve and fuel valve need to have a very small volume and secondly, monitoring of the gas channel pressure necessary to ensure detection of leakages is made very difficult due to high frequency oscillation excited from closing of the window valve.
It is also known in the art to use booster pumps and fuel pressure controlled fuel valves for injecting liquid gas, such as e.g. LPG. This concept has the problem associated therewith that the compressibility of LPG is rather large and dependent on pressure, temperature and gas composition. Hence, the delay between the actuation of the pressure booster and the actual gas injection is dependent on those parameters, which will make engine control, i.e. injection amount and particularly injection timing, very difficult. This is a significant problem, since injection timing is critical in compression-ignited engines.
There is therefore a need to provide a fuel supply system for LPG and similar low flashpoint fuels that is safe, inexpensive and provides accurate control of the timing of the fuel admission into the cylinders.